Wireless communication systems are widely deployed to provide various types of communication such as voice and data. These systems may be based on code division multiple access (CDMA), time division multiple access (TDMA), or some other multiple access techniques. A CDMA system provides certain advantages over other types of systems, including increased system capacity.
A CDMA system may be designed to support one or more CDMA standards such as (1) the “TIA/EIA-95-B Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System” (the IS-95 standard), (2) the standard offered by a consortium named “3rd Generation Partnership Project” (3GPP) and embodied in a set of documents including Document Nos. 3G TS 25.211, 3G TS 25.212, 3G TS 25.213, and 3G TS 25.214 (the W-CDMA standard), (3) the standard offered by a consortium named “3rd Generation Partnership Project 2” (3GPP2) and embodied in “TR-45.5 Physical Layer Standard for cdma2000 Spread Spectrum Systems” (the IS-2000 standard), and (4) some other standards. An example non-CDMA system is the GSM system.
Neighboring systems may employ one or more radio access technologies on one or more frequencies. Additionally, a system may have one radio access technology overlaying another. For example, portions of a GSM system may be overlapped with W-CDMA base stations. It may be desirable for a mobile station communicating on the GSM system to handoff to the W-CDMA system without dropping an active call. Or, a mobile station communicating on a W-CDMA system may handoff to a W-CDMA system on an alternate frequency.
To facilitate such handoff, a mobile station must periodically search for base stations on alternate frequencies and/or alternate radio access technologies. In some communication systems, a list of potential neighboring base stations is transmitted from a base station to a mobile station. This list is commonly referred to as a neighbor list. By limiting searches to those base stations on the neighbor list, a mobile station can reduce the amount of time required for such searching. However, when searching is required on an alternate frequency, the time available for such searching may be limited while an active call is in session.
In one example, certain time periods are allocated to allow for inter-frequency searching. During these time periods, referred to as compressed mode gaps, the mobile station is not required to transmit or receive on the serving frequency, so the mobile station may switch frequencies, and the active communication session does not suffer interference from the switch. However, since the performance of a system, including the throughput for an active communication system, as well as system capacity, relies in part on efficient use of available base stations, it is important that searching is performed efficiently during the gaps.
Efficient searching is also generally desirable in other communication modes, in addition to inter-frequency searching. Searching for W-CDMA base stations may be carried out using a three-step search process, well known in the art. A three-step search process may be employed to identify cells without any knowledge of the cell's scrambling code. However, a full three-step search may take a relatively long period of time. There may be situations where a full three-step search is inconvenient, or not practical. In many cases, information is known about one or more cells in the system. For example, the scrambling codes in a neighbor list are a subset of the total set of scrambling codes. A mobile station, having previously searched and identified a base station, will also know that base station's scrambling code. Thus, in these example scenarios, the mobile station has available known scrambling code offsets for use in searching. In general, system performance is increased whenever search time can be reduced. There is therefore a need in the art for reducing search time using known scrambling code offsets.